This chapter demonstrates a biogas renewable energy resource potential study for electric power generation from easily available biogas feedstock materials in four selected case study sites. Under this study, the site used in the model is a rural Kebele in Jama Woreda at 10.548° N, 39.33° E. The common biogas feedstocks considered under this study are animal slurry, human feces and jatropha byproducts whereas the biodiesel is considered from jatropha seed.
- anaerobic digestion
- biogas digester
Biogas is a byproduct of biomass which contains methane (CH4) and carbon dioxide (CO2) as a main gas component in a 3:2 ratio and it is produced through micro bacterial digestion processes under anaerobic conditions from a variety of organic material from animal, agricultural, industrial and domestic wastes . The biogas production level is depending on the ingredient level in the feedstock. For example; if the material consists of mainly carbohydrates, like glucose and other simple sugars and high-molecular polymers such as cellulose and hemicelluloses, the methane production is low. However, if the fat content is high, the methane production is likewise high (Table 1) .
Methane and other additional hydrogen compounds make up the combustible part of biogas. Methane is a colorless and odorless gas with a boiling point of −162°C and it burns with a blue flame. At normal temperature and pressure, methane has a density of approximately 0.75 kg/m3. Due to carbon dioxide being somewhat heavier, biogas has a slightly higher density of 1.15–1.25 kg/m3. Pure methane has an upper calorific value of 39.8 MJ/m3 (11.06 kWh/m3) (Table 2) .
|Substrate||HRT (days)||Solid concentration (%)||Temperature (°C)||Biogas yield (m3/kg VS)||Methane (%)|
|Mango processing waste||20||10||35||0.45||52|
|Mixed feed of fruit waste||20||4||37||0.62||50|
2. The biogas production process
Anaerobic digestion (AD) is a biochemical process during which complex organic matter is decomposed in absence of oxygen, by various types of anaerobic microorganisms. The result of the AD process is the biogas and the digestate. Biogas is a combustible gas, consisting primarily of methane and carbon dioxide. Digestate is the decomposed substrate, resulted from the production of biogas. If the substrate for AD is a homogenous mixture of two or more feedstock types (e.g., animal slurries and organic wastes from food industries), the process is called “co-digestion” and is common to most biogas applications today.
The process of biogas formation is a result of linked process steps, in which the initial material is continuously broken down into smaller units. Specific groups of micro-organisms are involved in each individual step. The simplified diagram of the AD process, shown in Figure 1, highlights the four main process steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The process steps quoted in Figure 1 run parallel in time and space, in the digester tank. During hydrolysis, relatively small amounts of biogas are produced. Biogas production reaches its peak during methanogenesis .
Methanogenesis is a critical step in the entire anaerobic digestion process, as it is the slowest biochemical reaction of the process. Methanogenesis is severely influenced by operation conditions. Composition of feedstock, feeding rate, temperature, water content, NH3 concentration and pH are examples of factors influencing the methanogenesis process.
Temperature for fermentation will greatly affect biogas production. The AD process can take place at different temperatures, divided into three temperature ranges: psychrophilic (below 20°C), mesophilic (30–42°C), and thermophilic (43–55°C). There is a direct relation between the process temperature and the HRT. The biogas production rate increases with increase the process temperature (Table 3).
|Thermal stage||Process Temperature||Minimum HRT|
|Psychrophilic||< 20° C||70–80 days|
|Mesophilic||30–42° C||30–40 days|
|Thermophilic||43–55° C||15–20 days|
In practice most modern biogas plants operate at thermophilic process temperatures because this process provides many advantages, compared to mesophilic and psychrophilic processes:
Effective destruction of pathogens
Fast grow rate of methanogenic bacteria at higher temperature
Minimization of biogas production period, making the process faster and more efficient
Improve digestibility and availability of substrates
better decomposition and utilization of solid substrates
Increase the chance to separate liquid and solid fractions
The metabolic processes in the production of biogas from different biomass feedstocks are hydrolysis, acidogenesis, acetogenesis and methanogenesis and their byproducts in the process is represented in the figure below.
In this study thermophilic biogas temperature process is chosen in order to get higher biogas output and to achieve this target flat plate collector can be used to maintain digester process temperature at 55oc.
3. Biogas plant
A biogas plant is a complex installation, consisting of a variety of elements. The layout of such a plant depends to a large extent on the types and amounts of feedstock supplied. Now there are several main types of biogas plants all over the world. Each time it is necessary to find the most suitable type in different case. Public acceptance, cost and energy efficiency are the main criteria to install biogas plant and efficiently utilize the biogas production. In smaller areas with scarcity of biogas feedstock or slurry to use low cost clay, concert or stone masonry made biogas digester.
Installation and operation of a biogas plant is a combination of environmental, safety, economic and technical considerations. Acquiring maximum methane output, by complete digestion of feedstock substrate, would require a long fermentation or digestion time of the material inside the biogas digester and a correspondingly large digester size. The ultimate goal of biogas production is getting the highest possible methane output and having justifiable plant economy. Biogas plants have the following main components and operate with four different process stages .
Process stages of biogas production:
Transport, delivery, storage and pre-treatment of feedstocks
Storage of digestate, conditioning and utilization
Storage of biogas, conditioning and utilization.
Main components of biogas plant:
Feedstock pre-storage tank
Substrate mixing Tank
Post storage tank
Gas holder tank and
The amount and type of available feedstock can determine the size, type and design structure of the biogas plant. The amount of biogas feedstock could determine the dimensioning of the digester size, storage capacities and CHP unit (Figure 2).
The CHP system utilizes the biogas either in heat or electrical energy. The properties of the combustible methane gas (like as shown in Table 4) will affect the operation of the CHP equipment. The combustion nature of the gas must be guaranteed, to prevent damage to the engines. Further treatment and enhancing chemical and physical properties of biogas even possible to use it for other utilizations like as vehicle fuel or in fuel cells application.
|1.||Lower heat value||LHV||≥4 kWh/m3|
|2.||Sulfur content||S||≤2.2 g/m3 CH4|
|3.||Hydrogen sulfide||H2S||≤0.15 Vol. %|
|4.||Chlorine content||Cl||≤100 mg/m3 CH4|
|5.||Fluoride content||F||≤50 mg/m3 CH4|
|6.||Dust (3–10 μm)||—||≤10 mg/m3 CH4|
|8.||Flow pressure||Pgas||20–100 mbar|
|9.||Gas pressure fluctuation||—||<±10% of set value|
|11.||Hydro carbon||HC||<0.4 mg/m3 CH4|
4. Design of the biogas plant
The design of the biogas plant includes the design of:
The gas Holder
Digester heat maintaining system
Siting of biogas plant
To calculate the scale of a biogas plant, certain characteristic parameters are used. These are:
Daily fermentation slurry feeding (Sd), which is an equal mixture of biogas feedstock (animal dung, human feces, poultry waste and jatropha byproduct) with water feed in to the biogas digester.
Retention time (RT), the time by which the fermentation slurry stays in the digester. It is about 2–5 weeks.
Digester loading (R). This parameter indicates the amount of biogas feedstock material per day is fed to the digester or to be digested. It can be measured in kg/m3/day.
Specific gas production per day (Gd), which depends on the retention time, the digestion temperature and the feed material.
4.1. Sizing of biogas digester and gasholder
The size of the digester—the digester volume (VD)—is determined by the length of the retention time (RT) and by the amount of fermentation slurry supplied daily (SD). The amount of fermentation slurry consists of the feed material considered in this study (e.g., cattle dung) and the mixing water.
4.1.1. Sizing of site-A biogas digester and gasholder
Daily average collectable biogas feedstock potential from cow dung, oxen dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct in this study in tons/day is 10.867 = 10,867 kg/day = 15.53 m3/day. Since the average density of animal slurry mix is 700 kg/m3.
Additional 15.53 m3/day water is required for proper digestion of biogas feedstock material to enhance biogas production.
HRT = 20 day, under thermophilic digestion temperature (55°C) the hydraulic retention time of the digestion process becomes short.
The volume of digester should be, VD = HRT × SD.
= 20 day × (15.53 × 2 m3/day) = 621 m3.
Therefore the size of the digester for site A could be 621 m3.
Where, VD = the size of the digester, HRT = hydraulic retention time, and SD is the amount of fermentation slurry (water + feedstock) feed in to the digester per day. Biogas yield in m3/kg of fresh biogas feedstock mix is 1736.4 m3/31850 kg = 0.054 m3/kg; the biogas production rate is 10,867 kg/day × 0.054 m3/kg = 588 m3/day. Therefore the size of gasholder should account this daily biogas production.
4.1.2. Sizing of site-B biogas digester and gasholder
Daily average collectable biogas feedstock potential from cow dung, oxen dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct of Site-B in tons/day is 9.253 = 9253 kg/day = 13.22 m3/day. Since the average density of animal slurry mix is 700 kg/m3.
Additional 13.22 m3/day water is required for proper digestion process of biogas feedstock material to enhance biogas production.
HRT = 20 day, under thermophilic digestion temperature the hydraulic retention time of the digestion process becomes short.
The volume of digester should be, VD = HRT × SD.
= 20 day × (13.22 × 2 m3/day) = 529 m3. Therefore the size of the digester for site-B is 529 m3. The biogas gas production rate is 9253 kg/day × 0.054 m3/kg = 501 m3/day. Therefore the size of gasholder should account this daily biogas production.
4.1.3. Sizing of site-C biogas digester and gasholder
Daily average collectable biogas feedstock potential from cattle dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct of site-C in tons/day is 8.82 = 8820 kg/day = 12.6 m3/day, Since the average density of animal slurry mix is 700 kg/m3.
Additional 12.6 m3/day water is required for proper digestion of biogas feedstock material to enhance biogas production.
The volume of digester should be, VD = HRT × SD, HRT = 20 day.
= 20 day × (12.6 × 2 m3/day) = 504 m3.
Therefore the size of the digester for site-C is 504 m3.
The gas production rate is 8820 kg/day × 0.054 m3/kg = 477 m3/day. Therefore the size of gasholder should account this daily biogas production also.
4.1.4. Sizing of site-D biogas digester and gasholder
Daily average collectable biogas feedstock potential of Site-D in tons/day is 3.091 = 3091 kg/day = 4.42 m3/day, since the average density of animal slurry mix is taken as 700 kg/m3. Additional 4.42 m3/day water is required.
The volume of digester should be, VD = HRT × SD, HRT = 20 day.
= 20 day × (4.42 × 2 m3/day) = 179 m3.
Therefore the size of the digester for site-D is 179 m3.
The gas production rate is 3091 kg/day × 0.054 m3/kg = 168 m3/day. Therefore the size of gasholder should account this daily biogas production.
4.2. Location of biogas plant
The next planning step in a biogas plant project idea is to find a suitable site for the establishment of the plant. The list below shows some important considerations to be made, before choosing the location of the plant: .
The site should be located at suitable distance from residential areas in order to avoid inconveniences, nuisance and thereby conflicts related to odors and increased traffic to and from the biogas plant.
The direction of the dominating winds must be considered in order to avoid wind born odors reaching residential areas.
The site should have easy access to infrastructure such as to the electricity grid, in order to facilitate the sale of electricity and to the transport roads in order to facilitate transport of feedstock and digestate.
The soil of the site should be investigated before starting the construction.
The chosen site should not be located in a potential flood affected area.
The size of the site must be suitable for the activities performed and for the amount of biomass supplied.
The site should be located relatively close (central) to the agricultural feedstock production (manure, slurry, energy crops) aiming to minimize distances, time and costs of feedstock transportation.
For cost efficiency reasons, the biogas plant should be located as close as possible to potential users of the produced heat and electricity.
The required site space for a biogas plant cannot be estimated in a simple way. Experience shows that for example a biogas plant of 500 kWel needs an area of approximate 8000 m2. This figure can be used as a guiding value only, as the actual area also depends on the chosen technology . Based on the above criteria of site selection of biogas plant, the location of the biogas plant for each site of the study area is chosen and the detail of it is found in the economic analysis section of the biogas plant in this paper.
5. Biogas potential
5.1. Biogas potential from jatropha
Various literatures show that methane yield of jatropha fruit hull is 0.438 m3/kg VS, and the VS is 76% of the TS of the jatropha fruit hull. Methane is 50% of the total biogas yield (1.153 m3/kg). The biogas yield of Jatropha seed presscake is approximately 1 m3/kg of presscake. The biogas yield of jatropha fruit hull is better than the seedcake . Based on the jatropha fact sheet given in Table 5, the biomass, biogas and methane yield potential of the jatropha byproduct is estimated in Tables 6, 7 and 8.
|Seed yield||dry ton/hectare/year||0.3||3.15||6||Position Paper on Jatropha Large Scale Project Development, FACT 2007|
|Fruit hull yield||dry ton/hectare/year||0.2||2.1||4|
|Rainfall requirements for seed production||mm/year||600||1000||1500||Position Paper on Jatropha Large Scale Project Development, FACT 2007|
|Oil content of seeds||% of mass||_||34%||40%||Jatropha bio-diesel production and use, W. Achten et al., 2008|
|Oil yield after pressing||% of mass of seed input||20%||25%||30%||Jatropha handbook, 2010|
|Presscake yield after pressing||% of mass of seed input||70||75||80|
|Energy content of Seed||MJ/kg||—||37||—|
|Biogas feedstock||Jatropha biomass, tons/year||Average jatropha biomass, tons/year||Biogas yield, m3/kg||Methane yield, m3/kg||Total biogas yield, m3||Average biogas yield, m3/year||Average methane yield, m3/year|
|Profile||Jatropha biomass, tons||Biogas yield, m3/kg||Biogas yield, m3||Methane yield, m3/kg||Methane yield, m3|
|Jatropha product||Jatropha oil (liter/year)||Jatropha biogas (m3/year)||Jatropha fertilizer (kg/year)||Jatropha biomass (ton/year)|
5.2. Biogas energy potential of the study area from animal dung
A wide range of biomass types can be used as substrates (feedstock) for the production of biogas from AD. The most common biomass categories used in biogas production are listed in Table 9 for this thesis work. To produce biogas from animal manure first we have to check whether we have animal livestock potential sufficient for biogas feedstock production or not. The following Table demonstrates the animal livestock potential for each sites of the study area.
|Animal livestock||Site-A||Site-B||Site-C||Site-D||Ave. no. of animal/HH||Total livestock in the study area|
The average fresh manure obtained from, cattle is 4.5 kg/day/head [1, 6, 7], donkey, horse and mule is 10 kg/day/head [6, 7], sheep and goat 1 kg/day/head [6, 7], and chicken is 0.08 kg/day/head [6, 7]. The average biogas yield of cattle, horse, mule, and donkey manure is 0.24 m3/kg DM [2, 3, 8] and pigs, sheep and goat is 0.37 m3/kg DM whereas chicken is 0.4 m3/kg of DM [2, 3, 8]. The dry matter content from the total mass of fresh animal manure and the proportion of methane from the total biogas production is summarized in Table 10 [2, 3, 9] (Table 11).
|Biomass source||Average fresh manure, kg/day/head||m3 biogas/kg DM||DM % fresh manure||Methane % biogas|
|Animal livestock||Ave. fresh manure, kg/day/head||Total no. of livestock in study area||Total fresh manure (ton/day)||Total DM (kg/day)||Biogas, m3/kg of DM||Total biogas, m3/day||Electricity production, kWh/day|
|Total animal manure biomass||36.183||6613||0.28||1850||2646|
For a given size of plant (rated gas production capacity per day) the amount of feedstock required can be estimated using the biogas yield data provided. The specific biogas consumption in biogas engines is 0.6–0.8 m3/kWh . This specific fuel consumption value can be used to calculate the requirement for biogas for power generation purposes. The expected biomass potential from animal manure of the case study area is 36.2 tons/day and its biogas production capacity is 1850 m3/day. Various literatures show that the collection efficiency of animal manure varies from country to country and region to region.
Most significantly the collection efficiency varies from 50 to 100% . Let as consider collection efficiency of 90% for cattle, donkey, mule, horse, pig and chicken manure, 50% for goat and sheep manure and 100% for human feces based on their difficulty of collecting it. Therefore the biomass potential available for biogas generation is estimated as follows.
The total collectable fresh animal manure biomass potential of the study area is estimated to be 30.235 tons/day and its biogas production capacity is 1398.3 m3/day (Table 12).
|Animal livestock||Ave. fresh manure, kg/day/head||Total no. of livestock in study area||Total collectable fresh manure, tons/day||Total collectable DM, kg/day||Biogas, m3/kg of DM||Total biogas, m3/day||Electricity production, kWh/day|
|Total animal manure Biomass||30.235||5235.8||0.27||1398.3||1998|
5.3. Biogas potential of the study area from human feces
Human feces are another feedstock for biogas production in the study area and the potential biogas production from human feces is discussed in this section. Feces are mostly made of water (about 75%). The rest is made of dead bacteria that helped us digest our food, living bacteria, protein, undigested food residue (known as fiber), waste material from food, cellular linings, fats, salts, and substances released from the intestines (such as mucus) and the liver (Table 13).
|Number of household||390||332||313||100||1135|
|Average Family per household||4.39 (5)||4.39 (5)||4.39 (5)||4.39 (5)||4.39 (5)|
One person produces on average 100–140 g of feces per day, the dry matter content of which is about 25% and its biogas yield of about 0.2 m3/kg DM . The total collectable fresh manure biomass potential of the case study area from humans is estimated to be 0.681 tons/day and its biogas production capacity is 34.05 m3/day. This figure accounts the collection efficiency of human excreta. Table 14 demonstrates the biogas potential of the study area from human feces.
|Live stock||Ave. fresh manure, kg/day/head||Total no. of population||Total fresh manure potential (ton/day)||Total DM (kg/day)||Biogas, m3/kg DM||Total biogas, m3/day||Electricity production, kWh/day|
5.4. Total biogas potential of the study area
The total biogas potential from Jatropha byproduct, Animal waste and human feces discussed above can be summarized in this section.
Taking the density of biogas 1.15 kg/m3 and calculating the gasification ratio (the mass of biogas produced per unit mass of feed stock consumed) of the biogas system. From Table 15 the mass of biogas feedstock consumed is 31,850 kg/day and the gas produced is 1736.4 m3/day. Therefore the gasification ratio of biogas feedstock mix is 1736.4 m3/31850 kg = 0.0545 m3/kg = 0.0626 kg/kg.
As we have seen from Table 15, animal manure is the major biogas feedstock constitutes which accounts 97% from the total biogas feedstock potential whereas jatropha byproducts and human excreta constitute 1 and 2% of the total biogas feedstock potential of the study area respectively. However, the share of biogas production from, animal manure is 82%, and human excreta is 2% but biogas production from jatropha byproduct is increase to 16% regardless of its low contribution to the biomass potential since the biogas yield of jatropha byproduct is high as compared to both animal and human manure and this can be summarized in Figure 3 given below.
|Animal Livestock||Ave. fresh manure, kg/day/head||Total no. of live stock||Total collectable fresh manure (ton/day)||Total collectable DM (kg/day)||Biogas, m3/kg DM||Total biogas production, m3/day||Electricity yield, kWh/day|
|Jatropha byproduct biomass||0.253||253||1.07||270||386|
5.5. Monthly variation of the biogas feed stock potential
The variation of jatropha byproduct feedstocks is assumed to be constant throughout the year and the potential biomass obtained from it was divided to each site regardless of the total house hold in each of the study area.
However, the biomass obtained from animal is highly depending on the availability and type of the animal feeding material. The animal feeding materials are varying in type and amount from month to month in the study area. In June and July there is enough root grass in addition to the usual animal food, let as consider this value as the annual average in ton/day (the data obtained by multiplying the biomass obtained per animal live stock in ton/day with the total number of animal live stock for each animal group in the district), as a reference frame. In January, February, and December there is excess dry agricultural farm grass for the animal food in the study area and assuming a 5% biomass resource increment is expected from the reference. March and April is a dry season and there is no enough food for the animal so considering a 5% biomass resource decrement from the reference. May, extremely drought month and August, animal grazing area are not permitted for animal food assuming a 10% animal based biomass resource drop is expected. From September to November there is excess animal food and a 10% biomass growth is assumed. Also assuming chicken manure and human feces are constant throughout the year. Taking in to account the assumption listed above the biogas feedstock potential month to month variation is presented in Tables 16–19.
The renewable energy potential of the site is estimated based on the primary data collected directly from the study area and secondary data obtained from various sources. The biogas feedstock mix potential of the study area is found to be 10.9 tons/day, 9.25 tons/day, 8.81 tons/day and 3.09 tons/day for Site-A, Site-B, Site-C and Site-D respectively with a gasification ratio of 0.0626 kg/kg. The study result shows that there is a sufficient biogas feedstock potential for all districts of the study area and the feasibility simulation result demonstrates there is an excess biogas after running a biogas generator in a hybrid system. The excess biogas left unused from a hybrid electric generating unit would go to biogas cooking application for the community cooking loads. Also, the biodiesel potential of the study area from Jatropha is estimated to be 18.5 m3/year.